1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or monitor and to an image forming method, more particularly to the redistribution of pixel value in the image forming apparatus or method.
2. Description of the Related Art
In image forming apparatuses it is frequently necessary to convert input image data to a resolution and tone scale capable of output by the image forming apparatus. Conventional methods employed for this purpose include dithering, density patterns, and error diffusion, but there are problems associated with these methods.
Dithering, which is used in general printing, and density patterns, which can be used to obtain halftone screen effects, are areal methods that obtain tone rendition by sacrificing spatial resolution. Dithering also tends to produce unwanted moire patterns when the input image is a halftone image.
Error diffusion can reproduce sharper edges than dithering and density patterns and is good at rendering text, but in rendering intermediate tones and gradations, it tends to generate dot patterns that the image forming apparatus cannot faithfully reproduce. Consequent problems include vanishing of low-tone image areas and collapse of gradations in medium- or high-tone image areas. These problems also occur with dithering, and with dispersed density patterns.
When the image forming apparatus is a high resolution electrophotographic printer, there are particular difficulties in reproducing isolated dots or small groups of dots, which occur frequently in all of the above methods. Further discussion can be found in Japanese Patent Application Publication No. 2002-271624.